the massachusetts senator is portraiting the former massachusetts governor who is mitt romney, and romney is using ron portman as his obama. paul steinhauser has more on their practice sessions. >> reporter: good morning, gary. you know, it's pretty obvious. one will definitely dominate this week in the race for the white hoe. >> i'm looking forward to the debates. >> reporter: and the first of the three showdowns comes wednesday night when president obama and republican nominee mitt romney face off in denver. both candidates have a lot on the line, especially the gop challenger, who a few days ago gave some insight into his debate preps. >> it's great to have senator rob portman. you know, he debates me from time to time. he is playing barack obama in these mock debates we have. i don't like him very much anymore, all right? he keeps on beating me up and i keep on going away shaking my head. >> reporter: romney flies to colorado monday to hold a rally that night before hunkering down for final preparations. the president heads to nevada sunday greeting voters that night and then goes beh